Silica aerogel is known as a material having small thermal conductivity and thermal insulation. Silica aerogel is useful as a functional material having excellent functionalities (e.g., thermal insulation), unique optical properties, and unique electric properties and is, for example, used among others as an electronic substrate material that utilizes the super-low permittivity characteristics of the silica aerogel, as a thermal insulation material that utilizes the high thermal insulating property of the silica aerogel, or as a light reflecting material that utilizes the super-low reflective index of the silica aerogel.
As a process for producing such silica aerogel, there is known a supercritical drying method by which alkoxysilane is hydrolyzed and polymerized, and the obtained gel compound (alcogel) is dried under the supercritical conditions of a dispersion medium: for example, Patent Literature 1 should be referred to. The supercritical drying method is a method for removing a solvent contained in the alcogel which comprises introducing the alcogel and the dispersion medium (the solvent for use in drying) into a pressurized container and applying to the dispersion medium, a temperature and pressure above its critical point to generate a supercritical fluid. However, since the supercritical drying method requires a high pressure process, capital investments in special devices and others that can endure the supercriticality are necessary, and also simultaneously, much time and effort are needed.
Therefore, a technique has been proposed that an alcogel is dried by employing a commonly used method which does not require the high pressure process. As such method, there is, for example, known a method by which monoalkyltrialkoxysilane and tetraalkoxysilane as raw gel materials are combined at a specific ratio to improve the strength of the gel to be obtained and it is dried under normal pressure: for example, Patent Literature 2 should be referred to. However, when such normal pressure drying is employed, the gel tends to shrink because of the stress resulting from the capillarity within the alcogel.